<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/vue/2.6.11/vue.min.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="num1">
        <select v-model="op">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
        </select>
        <input type="text" v-model="num2">
        <button @click="calc">=</button>
        <span>{{result}}</span>
    </div>
    <script>
        var app = new Vue({
            el: "#app",
            data: {
                num1: null,
                num2: null,
                op: null,
                result: null
            },
            methods: {
                calc() {
                    switch (this.op) {
                        case '+':
                            this.result = +this.num1 + +this.num2;
                            break;
                        case '-':
                            this.result = +this.num1 - +this.num2;
                            break;
                        case '*':
                            this.result = +this.num1 * +this.num2;
                            break;
                        case '/':
                            this.result = +this.num1 / +this.num2;
                            break;
                    }
                }
            }
        });
    </script>
</body>
</html>